Drying of sticky thermosensitive hydrous gels

ABSTRACT

A sticky water-soluble high-viscosity polyacrylamide hydrous gel is dried without significant decrease to its water-solubility by extruding the gel as paralleled cords upon a fast-travelling belt having a nonadherent surface and heating the cords with hightemperature gas until the surface of the cords is nontacky, and then discharging the cords upon a slowly travelling belt and completing the drying with low-temperature gas.

United States atet Zonis et al.

DRYING OF STICKY THERMOSENSITIVE HYDROUS GELS inventors: Meyer LouisZonis, Livingston; Girish Chandulal Shah, Wayne, both of N.J KennethWorden Saunders, Darien;

Michael Niall Desmond OConnor, Norwalk, both of Conn.

American Cyanamid Company, Stamford, Conn.

Filed: June 1, 1970 Appl. No.: 41,807

Assignee:

US. Cl ..34/12, 34/18 Int. Cl ..F26b 7/00 Field of Search ..34/9, l2, 18

[451 Jan. 1, 1972 [56] References Cited UNITED STATES PATENTS 2,045,3286/1936 Levey ..34/18 x 2,379,824 7/1945 Mummery ...34/23 x 2,443,4626/1948 Kimberlin, Jr. et al. ..34/9

Primary Examiner-John J. Camby Attorney-Evans Kahn [57] ABSTRACT Asticky water-soluble high-viscosity polyacrylamide hydrous gel is driedwithout significant decrease to its water-solubility by extruding thegel as paralleled cords upon a fast-travelling belt having a nonadherentsurface and heating the cords with high-temperature gas until thesurface of the cords is nontacky, and then discharging the cords upon aslowly travelling belt and completing the drying with low-temperaturegas.

12 Claims, 4 Drawing Figures PATENIED m 1 8 m2 SHEET 2 OF 4 m M w W MEYER LOU/S ZON/S G/R/SH CHANDULAL \SHAH KENNETH WOROE N SAUNDERS M/CHAE LN/AL L DESMOND O'CONNOR BY 2 ATTORNEY 'EPATENTED m 1 a 1972 SHEET 3 [IF4 //VVE/VTO/?5. MEYER LOU/5 ZO/V/S G/fi/SH CHA/VOULAL SHAH KENNETHWOROE/V SAUNDERS MICHAEL N/ALL OESMO/VD O'CONNOR ATTORNEYPATENTEHJAIIMITZ 3 634 944 SHEET u (1F 4 MEYER LOU/.5 ZON/S GIR/SHCHANOULAL 3H4 H KENNETH WOROE/V SAUNDERS M/OHAE L N/ALL' DES/MONOO'CONNOR ATTORNEY DRYING OF STICKY TI-IERMOSENSITIVE IIYDROUS GELS Thepresent invention relates to the continuous drying of sticky hydrouswater-soluble substantially self-supporting acrylamide polymer gels toapparently dry nonadhesive and grindable state without rendering themwater-insoluble, and to the grinding of said polymer to particulatefree-flowing state likewise without rendering the polymerwaterinsoluble.

Water-soluble acrylamide polymers (vinyl polymers which contain at least50 mol percent of acrylamide linkages) are currently produced on a majorindustrial scale and find use as wet and dry strengthening agents forpaper and as flocculants in the purification of water. The polymers(hereinafter termed polyacrylamides) are generally produced bypolymerization in aqueous medium of acrylamide alone or in admixturewith a molecularly minor amount of one or more water-soluble vinylmonomers copolymerizable therewith. The polymers are soluble in theaqueous medium. The polymers may be nonionic or they may be anionic orcationic (when they respectively contain anionic or cationicsubstituents). The immediate product of the polymerization is a clearhydrous gel so viscous as to be self-supporting i.e., so that a 1-inchcube thereof substantially maintains its shape when allowed to stand).The gel is very elastic and moreover it is intensely adhesive (about asmuch so as the glue of Scotch brand pressure-sensitive tape), andbecause of these properties it is very difficult to package in formacceptable to most consumers. Moreover, the gel is difficulty soluble inwater and typically takes many days to dissolve. Experience has shownthat the consumer prefers the polymer in apparently dry, particulatefree-flowing (ground) form, one reason being that the polymer in thisform dissolves comparatively rapidly in water. The gels are referred topossess low vapor and thermal permeability, and are very difficult todry. When the gel is dried in normal manner in bulk at hightemperatures, cross-linkages form on the surfaces of the gel which havebeen most strongly heated, causing portions of the polymer to becomeinsoluble. One mechanism by which this cross-linking takes place isthrough formation of imide linkages as illustrated by the theoreticalequation:

When the gel is dried at sufficiently low temperature to avoid formationof cross-linkages, an unduly long time and unduly large apparatus isrequired. Up to the present, for this reason, it has been consideredimpractical to dry the gel by the use of air (or other inert gas) alone.

The temperature at which the aforesaid cross-linkages start to form at asignificant rate varies from instance to instance depending principallyon the molecular weight of the polymer, the initial water content of thegel, the pH of the gel, the amount of monomer present, the molarproportion of the acrylamide units to any other units present in thepolymer. The temperature at which these cross-linkages form in anyinstance to produce more than a negligible amount of water-insolublematter (hereinafter termed thermal degradation point") can be readilyfound by laboratory trial. For polyacrylamide itself the temperature isabout 160 F. and for most acrylamide polymers the temperature is in therange of about l40l95 F. From the point of view of efficient drying,these temperatures are low.

In the past, it has been proposed to comminute the gel and dry the gelto grindable state by the use of air of moderate temperature, (e.g., airhaving a temperature of 300 F.) in the hope that the water content ofthe gel would maintain the temperature of the gel sufficiently low toprevent formation of cross-linkages in insolubilizing proportions whileproviding an adequately fast rate of passage through the drier. It wasfound, however, that the temperature of the gel doeslt was found,however, that the temperature of the gel does not remain uniform duringsuch drying but that the outer surfaces and particularly the comers ofthe comminuted gel reach the thermal degradation point sufficiently inadvance of the point at which the gel as a whole becomes dry so thatthese surfaces become thermally degraded and insoluble before the gel asa whole becomes sufficiently dry to be ground. The insoluble portions ofthe polymer appear as gelatinous particles which may be sufficientlylarge to be termed fish eyes (resembling tapioca particles) and usuallymust be removed by filtration before the polymer solution can beregarded as meeting commercial standards.

The discoveries have not been made that the aforementioned gels possessslight but significant adhesiveness to polytetrafluoroethylene (Teflon);that these gels, when extruded in the form of cords of more or lesscircular cross section of cn'tical area and contacted briefly with aninert drying gas having a temperature substantially above thedegradation point of the polymer in the gel, rapidly form a skin whichis flexible, uniform, and nonadherent; that polyacrylamide gel carryingthe aforementioned skin possesses substantially no adhesiveness forpolytetrafluoroethylene and can be readily stripped therefrom; that theresulting cords (having nonadhesive surfaces) can be automaticallyformed into thick gasperrneable mats which can be rapidly and safelydried by passage therethrough of an inert drying gas. By this means wehave found that the sticky hydrous gels can be dried to hard grindablestate in a compact drier at high throughput, that the resulting polymeris substantially completely (though slowly) water-soluble, and that thispolymer becomes rapidly watersoluble when ground to powder form, ashereinafter more particularly described.

The nonadherent skin which is formed at the end of the first heatingstage is temporary. The underlying body of the cords is highly hydrated,and when the cords are allowed to stand, water from the underlying bodydiffuses through the skin, rendering the surface substantially as tackyas it was before. In the process of the present invention, therefore,the second stage of drying is performed as a consecutive step, so thatsubstantially no hydration of the skin occurs.

The present invention, therefore, is broadly a continuous process forthe drying of a sticky, substantially self-supporting hydrous gel of awater-soluble thermosensitive polymer by extruding the gel as aplurality of parallel cords having a crosssectional area between about iand 500 mm. upon a travelling foraminous belt having a release surfacefor the polymer, passing a nonreactive drying gas having a temperaturefrom l00 F. below to 300 F. above the degradation temperature of thepolymer in the gel only until the surface of the cords has becomenonsticky, discharging the nonsticky cords from the belt, and contactingthe cords with a nonreactive drying gas having a temperaturesufficiently low that the aforementioned cross-linkages do not form.

In the process, the cross section area of the cords is critical. If thearea is much larger than 500 mm. the second stage of the drying willtake too long, the apparatus will not be compact, and the skin of thecords may become insoluble before the cords have been dried to grindablestate. If the area is much less than I mm?, the power requirements ofthe apparatus increase sharply because it is difficult to extrude atough rubbery gel in the form of small diameter threads, and since thevolume/surface area ratio will be small, there is danger that during thedrying, the temperature of the skin may be carried above the thermaldegradation point. As the result of a large number of trials we havefound it practical in the first stage to extrude the gel as cords havinga cross section area in the range stated and to convert the surface ofthe cords into a nontacky skin by contact with a hot current of air, andthen to form the cords into a thick mat and dry the mat with air havinga temperature not substantially in excess of the thermal degradationpoint. Being nontacky and substantially self-supporting, the mat whichis formed has an open structure permitting the passage of the drying airto every part.

The invention is described more in detail in the drawings, wherein FIG.1A is an elevation showing schematically the reactors in whichacrylamide polymer gel is formed and the end of the first travellingbelt on which the gel from the reactors is discharged;

FIG. 1B is an elevation partially broken away showing schematically theparts of the first and second belts on which the gel is dried, and thecomminution of the dried polymer to coarse particulate state;

FIG. 2 is a plan view, partly in section, of the two belts andcomminution section of the apparatus shown in FIGS. 1A and 1B; and

FIG. 3 shows partially in elevation and partially in section theapparatus for supply of hot inert gas to the second drying belt alongline A-A' of FIGS. 1B and 2.

In the figures the same reference numerals and letters designate thesame components.

In FIG. 1A the acrylamide polymer hydrous gel is formed inpolymerization chambers la and 1b working in parallel, and is suppliedthrough valves 2a and 2b working alternately through pipes 3 and 4 toscrew pump 5 provided with pipe 5a at its exit end for supply of specialadditives. The gel is supplied under pressure to header 6 extendingacross the width of travelling jointed steel belt 7 carrying releasesurface 7a where it is extruded in the form of cords 8 having a crosssection area of 1 mm. to 500 mm. and preferably in the range of2550 mm.

As is particularly shown in FIG. 1B, 2 and 3, belt 7 is carried forwardby drive roll 9 and idler roll 10 and carries the cords into cabinet 11where they are contacted with hot inert gas supplied by passage of thegas through conventional heating coils l2 and distributed by fan 13driven by motor in housing 14. The gel cords now carrying a nontackyskin are discharged by gravity from nonadherent belt 7 and fall upontravelling belt 15 carried forward by drive roll 16 and supported byidler roll I7. The speed of belt I5 is one-tenth that of belt 7. As aresult the cords form a mat 18 about l0 times the thickness ofindividual cords 8.

Belt 15 carries the mat into low-temperature drying cabinet I9 where themat is dried by passage therethrough of inert drying gas heated by coils20 and circulated by fan 21 driven by motor in housing 22 in the samemanner as the gas in cabinet 11.

The humidity and solvent content of the gas is controlled by bleeding ofgas through ducts 23a and 23b provided with dampers 24a and 24b leadinginto exhaust fan 25.

The mat of acrylamide polymer gel cords, now in hard, rigid fracturablestate, is continuously discharged from belt 15 upon cutting table 26where rotating chopper blades 27 fracture it into pieces 28 of grindablesize, which are collected in bin 29.

The pieces are then ground to a particle size range (e.g., 90 percentthrough 40 mesh) which permits the polymer to dissolve rapidly whenstirred with water. Any convenient grinder (for example, a hammer mill)may be used. In continuous operation the pieces entering the grinder areclose to their thermal decomposition point, and the grinding operationfurther elevates the temperature of the polymer. The grindingtemperature is, therefore, kept below the thermal decomposition point bythe introduction of cold gas, and we have found it most convenient tosupply such gas by spraying liquid nitrogen or carbon dioxide into thegrinding chamber.

The numerical constants of the apparatus vary from instance to instanceand do not conform to a simple rule. In each of the stages the principalvariants are the speed of the belt; the diameter, water content andcross section configuration of the cords on entrance; the temperature,velocity and relative humidity of the air in the two chambers; and thesupply (or nonsupply) of radiant (e.g., infrared) energy to the cords toprovide a penetrating heat. The constants cannot be easily predeterminedbut can be determined by trial, employing the data of the examples asthe start. However, we have found that in the second stage thetemperature of the heating gas can be as much as 50 F. above the thermaldegradation point without formation of an objectionably large proportionof insoluble matter in the polymer. In general, the larger thecross-sectional area of the polymer the lower should be the temperatureof the second stage drying air. The maximum temperature can be used inthe case of cords having cross-sectional areas in the bottom of therange (e.g., less than about 50 mmF). At the top ofthe range (e.g., inthe range I00 mm. to 500 mm?) the temperature of the drying gas shouldnot be more than a few degrees above the thermal degradation point.

In the process, the dwell time of the polymer in the second drying stageneed be no longer than is required to render the polymer convenientlygrindable, i.e., with a water content of5 to I5 percent. Insolubles tendto form in the polymer when the dwell time is extended, and there is nooffsetting advantage.

The process of the present invention is usefully employed with gels ofwater-soluble acrylamide polymers which contain a material selected fromthe group consisting of cyanamide, guanidine, hydrazine,mercaptosuccinic acid, succinimide, thioglycollic acid, urea, andmixtures of these materials as agents which inhibit the formation ofcross-linkages in the polymer. Without these materials, the driedacrylamide polymer product of the present invention generally contains atrace of water-insoluble material. With the above-mentioned materialspresent in effective amount (usually 0.02-2 percent on a dry basis), thepolymer product is generally completely soluble in water. If desired,the materials may be introduced by uniformly mixing them in dry powderedstate into the gel and allowing the gel to stand. The particles dissolveand become uniformly distributed through the gel. Alternatively, thematerials may be incorporated into the gel by dispersing the material insolution state throughout the gel and allowing a few minutes for theliquid to diffuse through the gel. Alternatively still, the materialsmay be introduced into the solution of monomeric material from which thepolymer is formed. Details are provided in copending application Ser.No. 878,883, filed on Nov. 21, 1967 by M.N.D. OConnor, The presence ofthese compounds in the gel during the drying step confer the addedbenefit of converting any monomeric acrylamide present to other andnontoxic form, as is disclosed in said application. Monomeric acrylamideis toxic, and to this extent the materials recited above have theproperty of acting as detoxifying agents.

The grinding step is facilitated by the presence of a uniformlydistributed amount of a water-soluble salt in the polymer. The saltincreases the throughput of the grinder without increasing powerrequirements on the one hand, and permits the output of the grinder tobe increased with less power than would be expected. Suitable salts forthe purpose include sodium sulfate, potassium sulfate, sodium acetate,disodium phosphate, sodium nitrate, aluminum sulfate and magnesiumsulfate.

The invention is further described by the examples which follow. Theseexamples are preferred embodiments of the invention and are not to beconstrued in limitation thereof.

EXAMPLE I The following illustrates a typical embodiment of the presentinvention performed in apparatus similar to that described in thedrawings.

An aqueous hydrous gel of polyacrylamide having a molecular weightofabout l0 million, a water content ofabout percent by weight, a thermaldegradation temperature of about [60 F., is delivered under a pressureof about lb./in. by a screw pump to an extruder similar to that shown inFIG. 2. The extruder produces a parallel array, about 10 feet wide, ofcords one-fourth inch in diameter and one-half inch apart. The extrudedcords are about as sticky as surgical adhesive tape, they are veryflexible, and resemble elastic band rubber. The cords fall withoutlosing shape upon a horizontal stainless steel belt feet wide and 2inches below the extruder) carrying a polytetrafluoroethylene fabricsurface and moving 10 feet per minute. This speed is slightly fasterthan the speed at which the cords leave the extruder. The cords adhereslightly to the polytetrafluoroethylene surface, so that the cords areslightly stretched and lie flat on the belt in parallel array. The cordsare carried on the belt into a drying chamber 50 feet long where theyare contacted with air at 330 F. flowing through the chamber at a speedof 300 feet per minute. 0n emerging from the chamber (dwell time 5.0minutes) the cords have a water content of about 70 percent by weight.They carry a nonadhesive skin but are still flexible, elastic, clear andglossy, having soft and sticky centers.

The cords are then discharged upon a horizontal stainless steel beltmoving 5.0 inches per minute and are carried in the form of a thickopen-structured mat about 4 inches thick into a drier chamber 50 feetlong where they are contacted with dry air at l60 F. On emerging fromthe chamber (residence time about 2 hours) the cords have a watercontent of 5-10 percent by weight and a diameter of about one-eighthinch. They are opaque, stiff, and uniformly hard and dry.

The mat is then passed under a rotary chopper where it is broken intopieces roughly one-half inch in largest dimension, which are fed into ahammer mill, the interior of which is maintained at 120 F. (well belowthe thermal degradation temperature of the polymer) by liquid nitrogensprayed into the interior through pipes passing through the sides of thegrinding chamber.

The product from the hammer mill has a particle size of 100 percent 1 8mesh, 56 percent 30 mesh and 18 percent 100 mesh. it dissolvescompletely in 30 minutes in water at one-half percent concentration at70 F. No fish eyes are visible in the solution. Filtration of thesolution shows that the product contains a trace (less than 0.5 percent)of insoluble matter.

EXAMPLE 2 The following illustrates the production of a similar driedpolyacrylamide powder in more compact apparatus resulting from the useof a gas for the second drying step which has a temperature above thethermal degradation point of the polymer.

The procedure of example 1 is repeated using the same gel except thatthe length of the heated zone in the second drying oven is decreased to35 feet, and the temperature of the second drying air is increased to190 F. The product is substantially the same as the product of examplei. Evidently, in the second drying stage evaporation of water from thegel kept the temperature of the gel below the thermal degradation pointof the polymer.

EXAMPLE 3 The following illustrates a method for the production of a dryacrylamide polymer which contains no detectable amount of insolublepolymer or other insoluble polymer.

The procedure of example I is repeated using the same gel except thatthe gel has a uniform dissolved content of l percent of urea based onthe weight of polymer therein, as disclosed in said copendingapplication.

The ground product dissolves completely in water at a concentration ofone-half percent and a temperature of 70 F. and the solution contains noinsoluble matter.

EXAMPLE 4 The following illustrates a process according to the presentinvention in which a polyacrylamide gel is dried and ground to a form inwhich it is rapidly and completely water-soluble and in which some ofthe amide substituents are converted to anionic (carboxylic) form.

The procedure of example 1 is repeated using the same gel except that0.07 mol of sodium hydroxide (as a l5 percent by weight aqueoussolution) per acrylarnide linkage in the polymer is metered into the gelas it leaves the screw pump. About 7 percent of the amide substituentsof the polyacrylamide hydrolyze during the subsequent mixing and dryingsteps, so that the product corresponds to a 93:7 molar ratio anionicacrylamidemcrylic acid copolymer. The product, after grinding, dissolvesrapidly in water at one-half percent concentration and 70 F. to form asolution which contains less than 0.5 percent insoluble matter.

EXAMPLE 5 The following illustrates the process of the present inventionon a gel wherein the polymer contains cationic substituents.

The procedure of example 1 is repeated except that the polymer in thegel which is dried is a 95:5 molar ratioacrylamide:2-(dimethylamino)ethyl methacrylate copolymer having amolecular weight of roughly 10,000,000. The product, after grinding,contains less than 0.5 percent insolubles.

EXAMPLE 6 The following illustrates the process of the present inventionwherein the gel when dried contains a water-soluble salt as agentimproving the grindability of the gel.

The procedure of example i is repeated except that sufficient of a 25percent solution of sodium sulfate is metered into the gel as it leavesthe screw pump to provide 15 percent of Na SO based on the dry weight ofthe polymer. The dry polymer is substantially the same, but is morereadily ground.

EXAMPLE 7 The following illustrates the process of the present inventionapplied to the drying of a preformed anionic acrylamide polymer gel withintroduction of supplementary polymerization catalyst to detoxify anymonomer present. The gel is similar to that of example 1, but thepolymer is a 90:10 acrylamide:acrylic acid copolymer having a molecularweight of about 5,000,000.

The procedure of example 1 is repeated, except that into the geltransport line between the transfer pump and the extruder is pumped asolution of alkali metal persulfate (K S O sufiicient to supply 50-80ppm. of the persulfate based on the dry weight of the polymer exceptthat in the first stage of drying the cords are contacted with air at265 F. for 9 minutes and in the second stage the cords (in matted state)are contacted with air at 180 F. for about minutes. The dry polymercontains 0.01 percent by weight of water-insoluble matter.

EXAMPLE 8 The following illustrates the procedure of the presentinvention wherein drying and a large amount of hydrolysis occur duringthe drying.

The procedure of example 1 is repeated except that sufficientconcentrated sodium carbonate solution is metered into the gel onexiting from the screw pump to provide 35 percent hydrolysis of thepolyacrylamide, and in the first stage the gel is contacted with air at265 F. for 21 minutes and in the second stage with air at F. for I06minutes. The product contains 9.8 percent water by weight, is readilygrindable, and contains no detectable amount of insolubles.

We claim:

1. Continuous process for drying a sticky water-soluble, substantiallyself-supporting hydrous gel of a water-soluble acrylamide polymerwithout rendering said polymer water-insoluble, which comprisesextruding said gel as a plurality of cords having a cross-sectional areabetween about I mm. and 500 min. upon a travelling belt having a releasesurface for said polymer; contacting said cords on said belt with anonreactive drying gas having a temperature from 100 F. below to 300 F.above the degradation point of the polymer in said gel only until anonsticky skin has formed on said cords; discharging said cords fromsaid belt; and contacting said discharge cords with a nonreactive dryinggas having a temperature not more than 50 F. above the thermaldegradation point of the polymer in said gel until said cords aresufficiently dry to be grindable.

2. A process according to claim 1 wherein the cross section area of thecords is in the range of 25 mm. to 50 mm.

3. A process according to claim 1 wherein the temperature of said firstgas is 50-l00 F. above the degradation point of the polymer in saidcords.

4. A process according to claim 1 wherein the polymer contains 0.02-2percent by weight (dry basis) of a material selected from the groupconsisting of cyanamide, guanidine, hydrazine, mercaptosuccinic acid,succinimide, thioglycollic acid, urea, and mixtures thereof, as agentinhibiting formation of cross-linkages in said polymer.

5: A process according to claim 3 wherein the cords are discharged fromsaid first belt upon a second belt having a speed about one-fifth to oneone-hundredth that of the speed of said first belt.

6. A process according to claim I wherein the temperature of said seconddrying gas is at about the thermal degradation point of said polymer.

7. A continuous process for converting an aqueous, sticky, substantiallyself-supporting hydrous gel of a water-soluble acrylamide polymer todry, particulate and readily water-soluble state, which comprises dryingsaid gel to grindable state by the method of claim 1, and grinding saidgel in an inert atmosphere having a temperature sufficiently low tomaintain the temperature of said polymer during said grinding below itsthermal degradation point.

8. A process according to claim 7 wherein the temperature of saidatmosphere is maintained sufficiently low by evaporation of liquidnitrogen.

9. A process according to claim 1 wherein the gel contains awater-soluble alkaline material as hydrolyzing agent for amidesubstituents therein.

10. A process according to claim 9 wherein the alkaline material issodium hydroxide.

1]. A process according to claim 7 wherein the gel contains awater-soluble inorganic salt as agent improving the grindability of saidgel.

12. A process according to claim 11 wherein the salt is sodium sulfate.

2. A process according to claim 1 wherein the cross section area of thecords is in the range of 25 mm.2 to 50 mm.2.
 3. A process according toclaim 1 wherein the temperature of said first gas is 50*-100* F. abovethe degradation point of the polymer in said cords.
 4. A processaccording to claim 1 wherein the polymer contains 0.02- 2 percent byweight (dry basis) of a material selected from the group consisting ofcyanamide, guanidine, hydrazine, mercaptosuccinic acid, succinimide,thioglycollic acid, urea, and mixtures thereof, as agent inhibitingformation of cross-linkages in said polymer.
 5. A process according toclaim 3 wherein the cords are discharged from said first belt upon asecond belt having a speed about one-fifth to one one-hundredth that ofthe speed of said first belt.
 6. A process according to claim 1 whereinthe temperature of said second drying gas is at about the thermaldegradation point of said polymer.
 7. A continuous process forconverting an aqueous, sticky, substantially self-supporting hydrous gelof a water-soluble acrylamide polymer to dry, particulate and readilywater-soluble state, which comprises drying said gel to grindable stateby the method of claim 1, and grinding said gel in an inert atmospherehaving a temperature sufficiently low to maintain the temperature ofsaid polymer during said grinding below its thermal degradation point.8. A process according to claim 7 wherein the temperature of saidatmosphere is maintained sufficiently low by evaporation of liquidnitrogen.
 9. A process according to claim 1 wherein the gel contains awater-soluble alkaline material as hydrolyzing agent for amidesubstituents therein.
 10. A process according to claim 9 wherein thealkaline material is sodium hydroxide.
 11. A process according to claim7 wherein the gel contains a water-soluble inorganic salt as agentimproving the grindability of said gel.
 12. A process according to claim11 wherein the salt is sodium sulfate.